1. Field of the Invention
The present invention relates generally to RF components, and particularly to RF components such as circulators and isolators.
2. Technical Background
Circulators and isolators are passive multi-port microwave device that are typically used in RF transmission line applications. A typical ferrite circulator includes three ports, and is generally referred to as a Y-junction circulator. In operation, when an RF signal is directed into a first port, the RF signal will be accessible via the second port in sequence, i.e., the port immediately adjacent the input port. The RF signal will be substantially attenuated and will not be available at the third port in the sequence, i.e., the port immediately adjacent to the second port on the other side of the first input port. On the other hand, if an RF signal is directed into the second port, it will be available as an RF output signal at the third port, but will not be available at the first port. Finally, if an RF signal is introduced at the third port, it will be available as an RF output at the first port, but not at the second port. A circulator, therefore, propagates RF power from one adjacent port to the next in a sequential, circular fashion. The RF signal circulation may be right-handed (RH) or left-handed (LH).
The circulation action in circulators/isolators is achieved by utilizing the “gyromagnetic effect” that is characteristic of ferrite materials. Ferrite materials have, in particular, specific magnetic properties which are mainly caused by spinning electrons. The spinning electrons have a magnetic moment and a mechanical moment. With the exposure of the ferrite material to an external magnetic field the magnetic moments can be aligned in parallel to the applied field. If all magnetic moments are aligned, the material is saturated. If another disturbing force, like an RF electromagnetic field, is applied to bring the electron spin out of alignment, a torque will act on the electron spin. The electron will then precess around the axis of the applied field with an angular frequency proportional to the applied field. The behavior of the material can be described mathematically using the Polder permeability tensor. The elements of the tensor are controlled by the RF frequency, the saturation magnetization of the material and the strength of the applied DC magnetic field. If the RF frequency is the same as the precession frequency, the ferrite material is operated at ferromagnetic resonance which also causes dissipation. Circulators and Isolators are generally operated with the magnetic biasing field adjusted to operate above or below ferromagnetic resonance.
When an RF signal is directed into the input port of the circulator, circulating phase shifted versions of the RF signal are induced within the ferrite discs. The degree of phase shift between counter circulating fields is a function of the strength of the DC magnetic field and diameter of the ferrite material. The circulator operates in accordance with the principles of superposition and constructive/destructive interference of counter-rotating RF waves. Using the example from above, when an RF signal is directed into the first port, the counter circulating RF signals are substantially in phase with each other at the second port, and therefore, they constructively interfere and reinforce each other. The amount of signal available at the second port is measured by what is commonly referred to as the insertion loss. In a properly functioning device the insertion loss is typically in the range of a few tenths of a decibel (dB). At the third port, the RF signals are out of phase with each other and substantially cancel each other. The term “substantially” refers to the fact that, in practice, the cancellation is not perfect and a residual signal may be detected. The amount of residual signal available at the third port, appropriately referred to as the “isolation,” is measured by the ratio of the residual signal and the incident signal. The isolation is typically between −25 dB and −30 dB.
A circulator may be configured as an isolator by terminating one of the ports with a “matched load” such that the complex impedance of the load is the complex conjugate of the output port impedance. As noted above, an isolator permits RF signal propagation between the two remaining ports in one direction only. RF power flow in the opposite direction is substantially inhibited. Now that the general operating principles have been briefly touched upon, a similarly brief description of the structure of a junction circulator is provided.
A junction circulator may be configured to include both electrical and magnetic circuit components and may be implemented using a stripline, microstrip or waveguide transmission configuration. The circulator includes a circuit portion having a flat center conductor that has three branches extending symmetrically outward from the central conductive portion. The three branches function as the ports of the circulator and are positioned 120° apart from each other. The center conductor is sandwiched between a pair of ferrite discs. The outer surface of both the top ferrite disc and bottom ferrite disc are in contact with ground planes to thereby form a stripline configuration. A permanent magnet is disposed over each ground plane. The permanent magnets apply a predetermined magnetic field to bias the ferrite discs in a predictable manner. A steel pole member may be inserted between each ground plane/magnet pair. The function of the steel pole member is to ensure that the biasing magnetic field applied to the ferrites is substantially uniform.
Those of ordinary skill in the art will understand that the operating frequency of circulators and isolators is determined by a number of factors like disc diameter, permittivity of the ferrite disc, biasing field level and circuit shape. The operating frequency for a biased-above-resonance (A/R) circulator is generally limited to approximately 4 GHz, while the operation frequency for biased below resonance (B/R) circulators extends up to 30 GHz. Below resonance circulators typically operate over broader frequency range than above resonance circulators.
Some of the drawbacks associated with the various circulators/isolators described above relate to manufacturability and functionality issues. Circulators and Isolators are typically implemented as drop-in or connectorized units. Below resonance (B/R) microstrip circulators are typically comprised of ferrite substrates. The electric circuit is implemented by sputtering the circuit material onto the ferrite substrate. A microstrip circulator operating above 10 GHz must be biased by a strong magnet that is typically comprised of a metallic alloy. The magnet should be aligned and placed precisely over the central conductive portion to prevent the RF field from being disturbed by the presence of a metal object disposed over the microstrip lines. Subsequently, the magnet is bonded to the ferrite surface using relatively precise bonding techniques. Accordingly, this approach is characterized by drawbacks that create significant challenges in a production environment. The other challenge associated with this design relates to the connection of the microstrip circulator with external circuitry. The aforementioned connection is usually accomplished using wire bonding techniques. One issue related to wire bonding techniques relates to discontinuities in the 50 Ohm transmission line impedance that are prone to the excitation of unwanted reflections in the transmission path. Moreover, below resonance micro-strip devices require manual labor both to mount them into an assembly and to provide the necessary RF connections.
What is needed is a surface mountable design that is amenable to automated pick and place manufacturing processes that substantially eliminate or reduce labor-intensive assembly.